1. Field of the Invention
The present invention concerns multichannel methods and devices to evaluate signals received with coils of a magnetic resonance tomography apparatus.
2. Description of the Prior Art
A magnetic resonance apparatus to examine patients by magnetic resonance tomography is known from DE 10314215 B4, for example.
Modern magnetic resonance systems operate with coils to emit radio-frequency pulses for nuclear magnetic resonance excitation and/or to receive induced magnetic resonance signals. A magnetic resonance system (MRT or MR) typically has a larger coil (known as a whole-body coil, also called a body coil or BC) that is normally permanently installed in the apparatus, as well as multiple small local coils (also called surface coils or LCs). To read out information from which images can be generated, selected regions of the subject or patient to be examined are read out with gradient coils for three axes (for example X, Y approximately radial to the patient, Z in the longitudinal direction of the patient). The spatial coding in magnetic resonance tomography is typically achieved with the use of a gradient coil system with three independently controllable, magnetically orthogonal gradient field coils. The orientation of the coding plane (“gradient field”) can be freely selected by superimposing the three freely scalable fields (in the three directions X, Y, Z).
As noted above, modern magnetic resonance systems normally operate with multiple different antennas (also called coils in the following) to emit radio-frequency pulses for nuclear magnetic resonance excitation and/or to receive the induced magnetic resonance signals. In contrast to the whole-body coil, the local coils serve to produce exposures with a very high signal-to-noise ratio (SNR). For this purpose the local coils are applied directly on the point (surface) of the patient at which the region to be examined is located. Given the use of such a local coil, in many cases transmission occurs with the whole-body coil (as the transmission coil) permanently installed in the magnetic resonance system and the induced magnetic resonance signals are received with the local coil (as reception coil).
In MR tomography today, images with high signal/noise ratio are normally acquired with such local coils (also called loops). The excited nuclei in the coil thereby induce a voltage that is then amplified with a low-noise preamplifier (LNA) that is relayed with its MR frequency to the receiver electronics via cable. Magnetic resonance scanners known as high field systems are also used to improve the signal-to-noise ratio (SNR) in high-resolution images. Their basic field strengths are presently 3 Tesla and higher. Since it should be possible to connect more coil elements (loops) to an MR receiver system than there are receivers present, a switching matrix (called an RCSS) is installed between the receiver antennas and the receivers (receiver circuits). This switching matrix routes the currently active received channels to the appropriate receiver. In order to be able to utilize the possibilities of parallel imaging (for example SENSE and GRAPPA, SMASH, etc.), reception coils with an increasingly higher number of channels are being developed.
It is conventional to use reception coils with 32-128 channels. The high number of reception channels places high demands on the reception chain. The high number of reception channels requires a high number of receivers (cost) and high computing effort in the image processing (computer time or costs for computing power). The use of the mode matrix is no longer reasonable from cost standpoints together with a frequency multiplexing method, since the mode matrix would have to be between the preamplifier and the mixer, which increases the configuration and costs of this module. Since the intermediate frequency receivers according to DE 10 2008 023 467 A1 are more advantageous than in the current product generation, the savings potential at the back end of the receiver chain (or RX chain)—for example in the form of an analog receiver—is markedly reduced while the use of the mode matrix together with the new intermediate frequency concept would turn out to be markedly more complicated technically and more expensive.
According to U.S. Pat. No. 7,098,659, a hardware mode matrix is known. This mode matrix combines adjacent receiver channels into combinations refined as modes. The mode matrix (MoMa) is a combination circuit composed of phase shifters and hybrids that combines the signals according to amplitude and phase so that N modes are obtained from N input signals from N coils. The first mode already contains the most important image information and offers the maximum SNR in the center of the patient. The use of higher modes offers increasing SNR in peripheral body regions and enables the application of parallel imaging techniques (for example SENSE or GRAPPA, SMASH etc.). Details regarding the mode matrix are described in U.S. Pat. No. 7,098,659, the disclosure of which is incorporated herein by reference. The mode matrix is used in modern products from the applicant in order to be able to operate the same coils with different receiver channel count.
An example of a previous solution is the following: 120 coil elements can be connected to an MR system. Although the switching matrix has 120 inputs and 32 outputs, only 8 receiver channels (receivers) are present. Solution: every 4 channels (for example) are combined into 4 modes by a mode matrix that is in the coil but lies after the preamplifier in the signal transmission direction. Therefore 8×4 modes are obtained from the 32 channels. If only the 8 basic modes (that are preferably the circularly polarized modes=CP modes) of these 8×4 modes are now read out, 8 reception channels suffice. This is the current prior art according to U.S. Pat. No. 7,098,659.
This technique allows a use of a higher-channel coil (=a coil with more channels provided) at a lower-channel receiver system (with limitations in the parallel imaging and given surface-proximal SNR). However, additional hardware is necessary for this in the coil. Therefore the aforementioned problem has not been solved in a satisfactory manner.
Moreover, such a hardware mode matrix entails the problem that, given mechanically flexible coils that can be deformed, the optimal SNR in the center of the patient body applies only for a deformed position of the coil (due to the relative position of the receiver loops to the reception field). In general, the combination of individual signals into modes is problematical if the antennas from which the signals arrive have no defined position relative to one another, or the antennas themselves can be deformed.